The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for forming a planarized metal layer in a semiconductor device. The present invention is an improvement over the invention which is the subject matter of the present inventor's copending U.S. patent application Ser. No. 07/585,218 filed on Sep. 19, 1990, the disclosure of which is hereby incorporated into this application by reference.
The metallization process is regarded by some as the most important aspect of semiconductor device manufacturing technology, since it increasingly determines yield, and the performance (e.g. speed of operation), and reliability of the devices, as the technology advances toward ultra large-scale integration (ULSI). Metal step coverage was not a serious problem with less dense prior art semiconductor devices, because of their characteristic features of larger geometries, contact holes having low aspect ratios (the ratio of depth to width), and shallow steps. However, with increased integration density in semiconductor devices, contact holes have become significantly smaller while impurity-doped regions formed in the surface of the semiconductor substrate have become much thinner. Due to the resultant higher aspect ratio of contact holes and larger depths of steps, with these current, greater-density semiconductor devices, it has become necessary, in order to achieve the standard design objectives of high-speed performance, high yield, and good reliability of the semiconductor device, to improve upon the conventional aluminum (Al) metallization process. More particularly, the utilization of the conventional Al metallization process in the fabrication of these current higher-density semiconductor devices, has resulted in such problems as degraded reliability and failure of the Al interconnections due to the high aspect ratio of contact holes and poor step coverage of the sputtered Al; increased contact resistance caused by silicon (Si) precipitation; and, degradation of the shallow junction characteristics due to Al spiking.
In an effort to overcome these problems of the conventional Al metallization process, various new processes have been proposed. For example, for preventing degraded semiconductor reliability caused by failure of the Al interconnection due to the high aspect ratio of contact holes and poor step coverage of the sputtered Al in Al metallization, the following processes have been proposed.
Japanese Laid-Open Publication No. 62-132348 (by Yukiyosu Sugano et al.), discloses a method for improving the conformity of a film formed over an abrupt step of a semiconductor device, which method comprises forming a metal wiring layer on the abrupt step (provided on a semiconductor substrate) and then thermally melting the wiring layer in such a manner as to planarize the metal wiring layer. Japanese Laid-Open Publication No. 63-99546 (by Shinpei Lijima et al.), discloses a method to improve wiring reliability and to enable the formation of a multilayer interconnection, wherein a metallic wiring layer is formed on a substrate having contact holes and steps, by means of heating and fusing the metallic wiring layer. More particularly, Shinpei Lijima et al. teaches a method for manufacturing a semiconductor device, which comprises the steps of forming multiple devices on a semiconductor substrate, depositing an insulation layer on the multiple devices, forming in the insulation layer contact holes reaching a predesignated portion of the device, forming a titanium nitride film on the surface of the insulation layer and contact holes, depositing a metallic wiring layer on the whole surface of the titanium nitride film and then heating the metallic layer so that it is fused and made to flow to planarize the surface of the metallic layer, and etching the metallic layer and the titanium nitride film according to a predesignated wiring pattern to form at least the first wiring layer.
In Japanese Laid-Open Publication No. 62-109341 (by Masahiro Shimizu et al.), for improving the reliability of a semiconductor device against wiring disconnections, there is suggested a method which comprises forming an aluminum conductive film having good coverage at a step, such as at a contact hole of an insulating film surface.
More particularly, Masahiro Shimizu et al. disclose a method for manufacturing a semiconductor device which comprises coating on a silicon substrate a solution containing liquid-phase aluminum (or aluminum compound), and then solidifying the same to form an aluminum conductive film.
According to all of the above methods, the contact hole is filled by means of melting and reflowing Al or an Al alloy. To summarize, in the reflowing step, the metal layer of Al or Al alloy is heated beyond its melting temperature, and the thusly melted metal is flowed into the contact hole to fill the same. This reflowing step entails the following drawbacks and disadvantages. First of all, the semiconductor wafer must be disposed horizontally so as to allow proper filling of the contact hole with the flowing melted material. Secondly, the liquid metal layer flowed into the contact hole will seek a lower surface tension, and thus, may, upon solidifying, shrink or warp, and thereby expose the underlying semiconductor material. Further, the heat treatment temperature cannot be precisely controlled and therefore, it is difficult to reproduce a given result. Moreover, although these methods may fill a contact hole with the melted metal of the metal layer, the remaining areas of the metal layer (outside of the contact hole area) may become rough, thereby impairing subsequent protective coating processes. Therefore, a second metal coating process may be required to smooth or planarize these rough areas of the metal layer.
It is also presently known that, for improving the reliability of the semiconductor by preventing degradation of the shallow junction characteristics due to Al spiking, a barrier layer can be formed in the contact hole formed on the semiconductor wafer. For example, in U.S. Pat. No. 4,897,709 (by Natsuki Yokoyama et al.), there is described a semiconductor device which includes a titanium nitride film (barrier layer) which is formed in a hole for preventing a reaction between the metal wiring layer and the semiconductor substrate. The titanium nitride film can be formed by a low pressure CVD method implemented with a cold-type CVD apparatus. The resultant film has excellent characteristics with good step coverage for a considerably fine hole having a large aspect ratio. After forming the titanium nitride film, a wiring layer is formed by a sputtering method using Al alloy.
As an alternative to melting Al or Al alloy for filling contact holes, and in order to improve the metal step coverage, a multiple step metallization process is disclosed in U.S. Pat. No. 4,970,176 (Clarence J. Tracy et al.). According to the above patent, a first portion of a predetermined thickness of a metal layer is deposited on a semiconductor wafer at a cold temperature; and then, the temperature is increased to a temperature of approximately 400.degree. C. to 500.degree. C., which allows the metal layer to reflow; and then, the remaining portion of the metal layer is deposited, or, alternatively, after the first metal layer is deposited, a second metal layer can be deposited, while increasing the temperature to the high temperature which allows for reflow of the metal layer. The reflow of the metal layer takes place through grain growth, recrystallization and bulk diffusion.
Ono et al. have disclosed that when the semiconductor substrate temperature is above 500.degree. C., the liquidity of Al-Si suddenly increases (Hisako Ono et al., in Proc., 1990 VMIC Conference June 11 and 12 pp. 76-82). According to the teaching of Ono et al., the stress of an Al-1%Si film changes abruptly near 500.degree. C., and the stress relaxation of t he Al-1%Si film occurs rapidly at that temperature. Additionally, the temperature of the semiconductor substrate must be maintained between 500.degree. C. and 550.degree. C. in order to fill the contact holes satisfactorily. This mechanism is different from the mechanism which facilitates reflow of the metal layer in the Tracy et al. ('176) patent.
One of the present inventors has an invention now pending in the U.S.P.T.O. entitled "A Method for Forming a Metal Layer in a Semiconductor Device," and filed as U.S. patent application Ser. No. 07/585,218. This invention relates to a method for forming a metal wiring layer through a contact hole in a semiconductor device, which comprises the steps of depositing a metal at a low temperature (below 200.degree. C.) and post-heating the deposited metal material at a temperature ranging from 80% of the melting point of the deposited metal material to its melting point temperature.
FIGS. 1A, 1B and 1C show a method for forming a metal layer according to the above invention. Referring to FIG. 1A, in which a process for forming a first metal layer is shown, a contact hole 2 is formed on the semiconductor substrate 10. Then, the substrate is put into a sputtering reaction chamber (not shown), in which a first metal layer 4 is formed by depositing the metal, (aluminum (Al) or Al alloy), at a temperature of 200.degree. C. or less and under a predetermined level of vacuum. This layer 4 has a grainy texture.
FIG. 1B illustrates the method of filling the contact hole. Referring to FIG. 1B, after the substrate structure obtained by the preceding process is moved to another sputter reaction chamber (not shown), without breaking the vacuum, heating is carried out for at least 2 minutes at a temperature of 550.degree. C., thereby filling up the contact hole with the metal. At this time, the pressure in the reaction chamber is preferably as low as possible so that the aluminum atoms have a higher surface free energy. In this manner, the metal can more easily fill the contact holes. The reference numeral 4a designates the metal filling the contact hole 2.
The heat treatment temperature range in the process shown in FIG. IB is essentially between 80% of the melting point of the metal and the melting point of the metal, and will vary according to the particular aluminum alloy or aluminum employed.
Since the metal layer is heat-treated at a temperature lower than aluminum's melting point of 660.degree. C., the metal layer does not melt. For example, at 550.degree. C., the Al atoms deposited by sputtering at a temperature below 150.degree. C. migrate upon heat-treatment at a higher temperature, instead of melting. This migration increases when the surface area is uneven or grainy due to an increase in energy among the surface atoms which are not in full contact with surrounding atoms. Thus, the initially sputtered, grainy layer exhibits increased atomic migration upon heat-treatment.
A process for forming a second layer 5 is shown in FIG. 1C. More particularly, second metal layer 5 is formed by depositing the remainder of the required total metal layer thickness at a temperature selected on the basis of the desired reliability of the semiconductor device. This completes the formation of the total (composite) metal layer.
According to the above method, the contact hole can be easily and fully filled up with a metal by using the same sputtering equipment used for the conventional thermal deposition method, and then annealing the deposited metal. Therefore, even a contact hole with a high aspect ratio can be completely filled.
However, when a void is formed in the contact hole or when the step coverage of the metal layer is inadequate, the contact hole cannot be filled up while maintaining the semiconductor wafer deposited with the metal layer at a predetermined temperature and vacuum level. Further, although a secondary metal layer is subsequently formed on the semiconductor wafer having a previously deposited primary metal layer, good step coverage of the contact hole cannot be assured, and the reliability of the manufactured semiconductor device is degraded due to this inadequate step coverage.
A contact structure consisting of pure Al deposited directly onto Si was adopted in the earliest stages of silicon technology. However, the Al-to-Si contact exhibits some poor contact characteristics such as junction spiking during sintering. The sintering step is performed after the contact metal film has been deposited and patterned. In the case of Al-Si contacts, such sintering causes the Al to react with the native-oxide layer that forms on the silicon surface. As the Al reacts with the thin SiO2 layer, Al2O3 is formed, and in a good ohmic contact, the native oxide is eventually completely consumed. Thereafter, Al diffuses through the resultant Al2O3 layer to reach the Si surface, forming an intimate metal-Si contact. Al must diffuse through the Al2O3 layer to reach the remaining SiO2. As the Al2O3 layer increases in thickness, it takes longer for Al to penetrate it. Thus, if the native-oxide layer is too thick, the Al2O3 layer eventually also becomes too thick for Al to diffuse through it. In this case, not all of the SiO2 will be consumed, and a poor ohmic contact will result. The penetration rate of Al through Al2O3 is a function of temperature. For acceptable sinter temperature and sinter times, the thickness of the Al2O3 should be in the range of 5-10.ANG.. Since the maximum Al2O3 thickness is of the order of the thickness of the native oxide that is consumed, an approximate upper limit to the allowable thickness of the native-oxide layer is fixed. The longer the silicon surface is exposed to an oxygen-containing ambient atmosphere, the thicker the native oxide will be. Therefore, surface-cleaning procedures in most contact processes are performed just prior to loading the wafers into the deposition chamber for metal deposition.
Aluminum absorbs from 0.5 to 1% silicon at a contact-alloying temperature between 450.degree. C. and 500.degree. C. If a pure Al film were heated to 450.degree. C. and a source of silicon were provided, then the Al would absorb silicon in solution until a Si concentration of 0.5 wt. % is reached. The semiconductor substrate serves as such a source of silicon, as silicon from the substrate enters the Al by diffusion, at elevated temperatures. If a large volume of Al is available, a significant quantity of the Si from below the Al-Si interface can diffuse into the Al film. Simultaneously, the Al from the film moves rapidly to fill the voids created by the departing Si. If the penetration of the Al is deeper than the pn-junction depth below the contact, the junction will exhibit large leakage currents or even become electrically shorted. This phenomenon is referred to as junction spiking.
For alleviating the problem of junction spiking at the contacts, Si is added to the Al film as it is deposited. Aluminum-silicon alloys (1.0 wt. % Si) have been widely adopted for manufacturing the contacts and interconnects of integrated circuits. The use of aluminum-silicon alloys instead of pure Al may alleviate the problem of junction spiking, but unfortunately, it causes another problem. More particularly, during the cooling cycle of the annealing process, the solubility of silicon in the Al decreases with decreasing temperature. The aluminum thus becomes supersaturated with Si, which cause nucleation and outgrowth of Si precipitates from the Al-Si solution. Such precipitation occurs both at the Al-SiO2 interface and the Al-Si interface in the contacts. If these precipitates form at the contact interface to create n+Si, an undersireable increase in contact resistance results. In addition, a large flux-divergence in the current is produced at locations where n+Si precipitates larger than approximately 1.5.mu. are formed. This can lead to early failure of the conductor due to an electromigration-induced open circuit condition.
FIG. 2. illustrates Si precipitates formed on the surface of the semiconductor substrate after metallization. Obviously, these Si precipitates should be removed. These Si precipitates have hitherto been removed by ashing, overetching or wet etching, or by using an etchant including a radical which may remove the precipitates from the substrate. However, when depositing the metal layer at a high temperature, the Si precipitates cannot be easily removed. When the Si precipitates are removed by overetching, the images thereof are transmitted to an underlying layer, and these images remain after the overetching. Thus, the quality and appearance of the surface of the semiconductor substrate remains poor.
Based upon the above and foregoing, it can be appreciated that there presently exists a need for a method for forming a planar metal wiring layer in a semiconductor device, which overcomes the above-described shortcomings and disadvantages of the presently available processes. The present invention addresses and fulfills this need.